This invention relates in general to alunite processing techniques and in particular to methods of producing alumina potassium sulphate from alunite.
The method of the invention is particularly applicable for processing alunite ores the rubbish content of which contains active forms of silicon compounds.
As it is commonly known, when alunite ores are treated with a solution of caustic alkalis the silicon compounds mentioned above are liable to form alkali-insoluble aluminosilicates which are discarded into slime and lost as waste. This adds to the loss of alkalis and aluminium and, accordingly, reduces the yield of alumina won from alunite.
In widespread use at the present is a method of producing alumina and potassium sulphate from alunite ores, involving the following steps: treating alunite ore, either crude or preliminarily roasted, with a solution of caustic alkalis at 85.degree. to 95.degree.C until aluminates and potassium and sodium sulphates are formed in the solution; separation of slime from the solution as a solid residue; evaporation of the solution until there is precipitation of a mixture of potassium and sodium sulphates; separation of the precipitate from the solution containing mainly sodium and potassium aluminates; treating the precipitated sulphates with a caustic potassium solution to form potassium sulphate; hydrolytic decomposition of sodium and potassium aluminates in the solution until aluminium hydroxide is formed; and the production of alumina by roasting aluminium hydroxide.
The cardinal disadvantage inherent in the known method is the aforementioned loss of alkalis and aluminium due to the formation of alkali-insoluble aluminosilicates discardable along with slime. As practical experience has shown, depending upon the amount of active forms of silicon compounds contained in alunite ores, the losses of alumina in slime amounts to as high as 15-20 percent of the total Al.sub.2 O.sub.3 content in the ore, and those of alkalis, to 150 kg per metric ton of commercial alumina.